Collagen arginine cross-links

The basic structure of collagen consists of the protofibril—a triple helix consisting of two identical a chains, as they are called, and one different chain. The a i chain of calf or rat hide consists of 1 052 a-amino acid residues, of which 1 001 are triplets of the general structure (gly-X-Y). Here, X may be proline, leucine, phenyl alanine, glutamic acid, and Y is mostly hydroxy-proline or arginine. Collagen is the only protein to contain hydroxyproline. This imino acid residue, however, is only formed after the protein biosynthesis. The triplets are, in turn, joined in sequences of polar and apolar regions. Telopeptides, peptide structures without triplet structure, occur at the N- and C-terminal triplet structures. The telopeptides are rich in lysine and they account for the intra- and intermolecular covalent cross-links. 

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